Surimi is a generic term for minced fish that has been processed to remove bones, fish oil and fish flavor. Typically, surimi is prepared by the fresh water leaching of mechanically deboned fish muscle. The leached muscle, after dewatering, yields a light-colored, bland proteinaceous material comprising essentially myofibrillar (contractile) protein, or surimi.
Conventionally, surimi is produced from fish, such as Alaskan pollack, however surimi derived from poultry, pork and beef also are known. With respect to fish, surimi can be prepared at sea or on shore. After production, surimi usually is frozen into blocks, typically about 20 kilograms in weight, and is stored for periods of weeks to months at freezing temperatures before use.
Frozen surimi is an intermediate protein source used to produce various foodstuffs, such as, for example, imitation crab legs, imitation scallops, kamaboko (fish jelly) and imitation lobster. To produce a surimi-based foodstuff, the bland-tasting surimi first is thawed, then salt is added to the thawed surimi during comminution of the surimi to a paste. The salt solubilizes the myofibrillar protein. Next, starch, non-muscle proteins (e.g., egg white, soy, whey), flavorings and colorants are added to the surimi paste. The resulting paste is formed into a desired shape (e.g., a crab leg), then heated to produce any of a range of foodstuff shapes and textures. The amount of surimi in such foodstuffs can range from 20% to 80% by weight of the foodstuff.
Fish species harvested to produce surimi often are available only far from shore and only during a short harvesting period. By necessity therefore, some surimi is prepared at sea, and is frozen on shipboard to preclude spoilage before use. A majority of the surimi that is processed on shore is also frozen because most fish species are harvested during a short harvesting period, but the demand for surimi is constant throughout the year. Therefore, a sufficient amount of surimi produced on shore or at sea must be frozen to accommodate this year long demand.
As previously stated, surimi comprises essentially myofibrillar proteins. Fish myofibrillar proteins are very susceptible to freeze-induced denaturation. Similar myofibrillar proteins derived from beef, pork and poultry also are subject to freeze-induced denaturation, but to a lesser extent than fish myofibrillar proteins. To overcome this susceptibility to protein denaturation, the refined myofibrillar component of fish muscle, i.e., surimi, is admixed with a cryoprotectant prior to freezing the surimi. A cryoprotectant is a chemical compound, or combination of compounds, that prevents significant protein denaturation and therefore imparts long-term storage stability to the frozen surimi. This long-term storage stability in turn ensures good protein functionality to allow use of the surimi in the manufacture of foodstuffs.
The term "functionality" refers to the specific attributes a food processor considers in adopting a protein source, like surimi, for use in a foodstuff. Functionality often is measured in terms of: (1) water binding ability to control water loss during storage, increased cook yield and increased juiciness; (2) fat binding ability to prevent fat separation during cooking; (3) texture; (4) gelation temperature and strength; (5) impact on appearance; and (6) impact on flavor and odor. As will be demonstrated in more detail hereinafter, functionality can be expressed quantitatively as gel-forming potential, which is manifested physically as texture formation and water-binding ability.
In the absence of a cryoprotectant, a surimi stored at freezing temperatures for extended periods has a decreased functionality. The freezing process causes ice crystal formation which results in dehydration of the myofibrillar protein, a pH decrease, and a change in salt concentration. These three effects, in addition to various hydrophobic interactions, denature and/or aggregate the frozen myofibrillar protein of surimi. In addition, the longer the surimi is frozen, the greater is the degree of protein denaturation.
Therefore, a cryoprotectant is added to the surimi to protect frozen surimi from a loss in functionality due to protein denaturing. A cryoprotectant must be intimately associated with the protein molecules to prevent denaturing. Therefore, cryoprotectants are useful in a minced product, like surimi, and are incorporated into the minced product before the product is frozen.
A cryoprotectant used to prevent significant denaturation of proteins in frozen surimi preferably meets several criteria. It is especially important for a cryoprotectant to maintain protein functionality during extended frozen storage of surimi, and thereby allow use of the surimi in the manufacture of foodstuffs. It is also desired that a cryoprotectant be relatively inexpensive, readily available, nontoxic, low in taste, water soluble, have good functional effects, and not appreciably brown the surimi during a cooking or heating process. Various proposed cryoprotectants have not met with commercial success because of their failure to meet one or more of these criteria. Such materials include: carbohydrate compounds, like mono- and di-saccharides; sugar alcohols; low molecular weight polyols; amino acids; carboxylic acids; triglycerides; hydrogenated glucose syrups; surfactants, such as polyoxyethylene sorbitan esters and sucrose esters; and quaternary amines. Other cryoprotectants and mechanisms of cryoprotection are discussed in G. A. McDonald et al., "Carbohydrates as Cryoprotectants for Meats and Surimi", Food Technology, March, 1991, pp. 150, 152-154, 156, and 158-159.
Presently, the standard, most widely used cryoprotectants for surimi are sucrose and sorbitol, either alone or in combination. In addition, a relatively small amount of a polyphosphate, such as sodium tripolyphosphate, is conventionally added to the surimi as a synergist to increase the cryoprotective effect of sucrose and/or sorbitol. Sucrose and sorbitol are the cryoprotectants of choice for surimi, and especially for Alaskan pollack surimi, because these cryoprotectants are readily available, relatively economical, and importantly, have a low tendency to cause Maillard browning when a surimi-based foodstuff is cooked or otherwise heated. Maillard browning is the well-known result of a reaction between a reducing sugar and a protein to produce brown pigments. A cryoprotectant that effectively resists browning the surimi is very important with respect to the bright white kamaboko products commonly served by the Japanese, the largest consumers of surimi.
Sucrose and sorbitol, however, add a definite sweet taste to surimi. Sucrose alone is a useful cryoprotectant but imparts too sweet of a taste to surimi. Therefore, sucrose usually is combined with sorbitol, in about a 1:1 weight ratio, and the mixture is used as a cryoprotectant for surimi. The sucrose-sorbitol mixture still imparts a perceptible sweet taste to surimi which is objectionable in many surimi-based foodstuffs. Therefore, it would be useful to provide a cryoprotectant: (1) that maintains the functionality of proteins in frozen surimi at least as well as sucrose and sorbitol, (2) that has a low tendency to cause Maillard browning during storage of surimi at freezing temperatures and during heating of a surimi-based foodstuff, and (3) that also possesses a low degree of taste.
In addition to sucrose and sorbitol, other specific cryoprotectants added to surimi include lactitol, maltose, fructose, lactose, mannitol, xylitol, lactilose, isomalt, maltitol, maltodextrin and various edible gums. Another cryoprotectant for surimi is polydextrose, a polymerized glucose which is a nonsweet, low calorie hydrolyzed starch bulking agent disclosed in Lanier et al. U.S. Pat. No. 4,572,838, and usually is used in combination with sorbitol. Yamamoto et al. U.S. Pat. No. 5,028,444 also discloses a composition consisting essentially of sodium bicarbonate, calcium citrate and calcium lactate that can be added to surimi, prior to freezing and in addition to a cryoprotectant, to improve the functionality of the frozen surimi.
Dextrose, in its conventional form of flat platelets, has also been used as a cryoprotectant for surimi in an attempt to reduce the sweetness imparted to surimi by sucrose. Dextrose, however, is also a reducing sugar and contributes to Maillard browning during cooking or heating of a surimi-based foodstuff. Therefore, reducing sugars are considered unsuitable cryoprotectants for a surimi that is processed into a light or white-colored foodstuff.
Because cryoprotectants currently added to a surimi either impart an objectionable sweet taste and/or significantly contribute to Maillard browning of the surimi-based foodstuff, there exists a need for an improved cryoprotectant that maintains the functionality of proteins in a frozen surimi, does not contribute significantly to Maillard browning during storage at freezing temperatures and during heating, and is bland in taste. The present invention is directed to cryoprotectants that provide a surimi having good functionality, that resist Maillard browning and that have a bland taste.